Field of the Invention
The invention relates to a spacer for a fuel bundle of a boiling water reactor, having cells formed by inner webs disposed in crosswise fashion and by outer webs surrounding the inner webs in the form of a frame. In the case of a fuel assembly for a boiling water reactor, a number of fuel rods are combined to form a fuel bundle in a fuel assembly channel. A number of spacers serving to guide the fuel rods are disposed over the fuel rod length. During reactor operation, water flows as primary coolant in the boiling water reactor from below into the fuel assembly and flows upward through the latter. The heat dissipated by the fuel rods heats the water as a result of which the latter partially evaporates. There is thus a two phase mixture of water and steam in the upper region of the fuel assembly. At the exit from the fuel element, the steam fraction is usually at approximately 60%.
There are special requirements placed on the configuration of a fuel assembly because of the two-phase flow typical of the boiling water reactor. Next to the observance of a suitable neutron moderation, this relates, in particular, to ensuring that the cooling of the fuel rods is sufficient, that is to say, adequate heat dissipation from the surface thereof. It must be ensured that the surface of the fuel rods is wetted with an adequate quantity of water. Specifically, this is a precondition for the so-called “nucleate boiling” in which steam bubbles are produced locally which propagate and finally burst. Heat is transferred in this case substantially through an annular zone below the bubble. However, there is the risk of so-called “foam boiling” because of the high steam quality in the upper region of the fuel element. This is understood as the effect that a water film located on the fuel rod surface boils off over the area without forming bubbles, something which finally leads to the fact that the fuel rod is no longer wetted with water in this region. The lesser degree of cooling thereby engendered raises the temperature sharply in the fuel rod and can lead to damage in the fuel rod.
It is known that there flows along the inner wall of the fuel assembly channel a coolant film whose steam quality increases in the upper region of the fuel bundle. It is known to use the cooling potential of the coolant film by leading coolant radially inward into the fuel bundle in the region of a spacer. A measure with the aid of which this can be achieved is described in U.S. Pat. No. 4,999,153. Grooves, so-called flow trippers running transverse to the flow direction of the coolant are worked into the inner wall of the fuel assembly channel. These serve the purpose of turbulently mixing the coolant flow at the inner channel wall. Positioned downstream of the grooves in the flow direction are discharge elements that project from the outer side of the outer webs and, as it were, scrape the water film turbulently mixed by the grooves from the inner channel wall and redirect it via a flow opening into the interior of the fuel bundle. In the case of a spacer known from Japanese Patent JP 05323073 A, a similar effect is achieved with the aid of outer webs penetrated by inflow openings.